Sourdough starter is a mixture of flour and water that cultivates yeast from the environment in a form that can be used for baking. If you can get a little sourdough starter from a friend or bakery you could just continue feeding the starter to keep it alive. If you need to make sourdough starter from scratch, then you can use this very rough guide. You can also follow a more detailed recipe at King Arthur's Sourdough Starter recipe

1. Mix the flour and the water very well, let rest for a day.
2. The second day, discard 50 g of the starter and add 50 g of water plus 50 g of flour. Mix well and let it rest for another day.
3. The third day, discard 100 g of the starter and add 50 g of water plus 50 g of flour. Mix well and let it rest for another day.
4. Days four to seven, repeat Step 3 till you starter is active, bubbly. It has to at least double in volume in between feedings.
5. On baking day, discard enough to KEEP 100 g of starter. Then, add 100 g of water and 100 g of flour. Mix well and let rest until starter doubles in volume. Use it immediately for your sourdough bread.